1. Field of the Invention
The present invention relates to apparatus for controlling the concentration of toner in the toner system of an electro-photographic apparatus. More specifically, the invention relates to a toner concentration sensor circuit for the apparatus.
2. Background of the Invention
Many prior art devices and apparatus have been employed in the past for detecting toner concentration so as to maintain the resulting copy uniform as to density and definition.
In one arrangement a pair of opposing glass plates provided with facing metallized surface is arranged so that toner passes between the plates which are electrically charged. The charge causes the toner to be attracted to the plates. An optical sensor, causes light to pass through the plates, the amount of toner being sensed is proportional to the amount of light received by the sensor-receptor. The sensitivity of this type of device is fairly low however, and detection is performed on the fly, i.e. as the toner is passing between the plates.
Still another apparatus employs "white" carrier beads with an optical sensor. The amount of "white" light reflected to the receptor varies with the amount of toner thereby enabling a determination to be made as to the concentration of the toner in the apparatus.
Each of the known types of toner concentration sensing devices is useful but no single device provides complete adequate and efficient signal output to prevent deterioration of the output copy. Thus, none of the prior art apparatus is capable of performing completely efficiently and adequately to provide copy whose density, definition and clarity remains constant throughout its period of use.
In order to maintain the optical density of the output copy constant in a high output device it is necessary to control the toner concentration in the printer or copier. High volume printer/copiers must operate more or less continuously and substantially unattended and therefore require some means for automatic control of the toner concentration. Many low volume copiers do not require automatic density control since the number of copies per day is sufficiently small that toner can be added manually by the operator as needed.
In application Ser. No. 429,861 "Toner Concentration Sensor for Electro-Photographic Apparatus" to Richard C. Fedder, filed Sept. 30, 1982, issued as U.S. Pat. No. 4,452,174 on June 5, 1984 assigned to the assignee, hereby incorporated by reference, apparatus was described which provided a simple, low cost, efficient and reliable means for controlling the toner concentration so as to provide a clean, clear, crisp, uniformly dense output copy. The basis for this apparatus is that studies have shown that the permeability of the developer mix changes with toner concentration with a higher permeability at lower toner concentration. This effect results from use of a two component developer (i.e. toner system) comprising a mixture of magnetic ferrite or steel carrier beads together with nonmagnetic toner particles. It is well known in the art of electro-photography that the toner particles and carrier beads take on the opposite sign of triboelectric charge when the mix is mechanically agitated. For example, the toner particles may become charged negatively due to rubbing against the carrier beads while the carrier beads become positively charged.
After a certain amount of mixing each carrier bead has many of the smaller negatively charged toner particles clinging to the surface of the bead. This coating of toner particles keeps the carrier beads slightly separated in the mix. With fewer toner particles on each carrier bead, the beads can come into closer proximity. In the extreme case when the toner concentration is zero, the carrier beads will come into direct contact with each other.
A variation in permeability is associated with the variation and separation of the carrier beads. Thus, when no toner is present while in a magnetic field, the carrier beads will hang together in a chain with no toner particles therebetween. The permeability along such a chain will be relatively high. Conversely, when toner particles are introduced into the mix the carrier beads will be separated somewhat by the toner particles. Thus, the permeability of the chain will be lower because of the intervening toner particles.
Ser. No. 429,861 takes advantage of the foregoing phenomenon to solve some of the problems associated with toner sensing. A modular, demountable, toner concentration sensor assembly is provided wherein a toner cartridge carrying a fresh supply of toner material is demountably insertable into the assembly so as to bring the fresh toner into the original developer mix and wherein a portion of developer mix is metered off the magnetic brush or application roller of the apparatus into an inductor surrounding a chute-like, open ended canister. An electromagnet is used to close the chute by immobilizing the developer mix within the chute while a signal output is taken from the inductance and applied to a sensing circuit to determine the quantity of toner to stabilize the mix concentration or bring the concentration to a predetermined level for copy printing output. An interconnected mechanism operably associated with the toner cartridge replenishes the toner on command from the system software as a result of a comparison between toner concentration levels and a selected concentration required for clear, clean, crisp printing/copying.